The development and secretory activity of the pituitary gland of cichlids, and a comparison of the meta-adenohypophysis with that of other teleosts

Abstract

The structure and histology of the pituitary gland of two species of teleost, Herichthys cyanoguttatus and Cichlasoma nigrofasciatum, is described. Seven cell types can be recognised. Two groups of P.A.S.+ve, A.F.+ve cyanophils in the meso-adenohypophysis are associated with the production of the thyrotrophic and gonadotrophic hormones; it is suggested that the P.A.S.+ve acidophils in the metaadenohypophysis produce a chromatophorotrophic hormone. The blood supply of the pituitary in cichlids of different ages has been studied. The development of the pituitary of Herichthys and the time at which the cell-types become histologically differentiated is described; comparisons are drawn with the developing pituitary of Cichlasoma. Histological differentiation of the first thyrotrophs coincides with the initial secretion of colloid in the thyroid but the results of experiments in which specimens of Herichthys were reared in thiourea solution indicate that pituitary influence on thyroid activity is not established until a later stage. Thiourea treatment from an early age affects not only the development of the thyroid and thyrotrophs but also that of the swim-bladder, melanophores and skin. The mechanism of these disturbances is discussed. The effect on the pituitary of long-term adaptation to both white and black backgrounds has been studied in several species ofteleost and in Rana. Chromatic adaptation is accompanied by cytological changes in the pars intermedia of Rana, and in certain cells in the meta-adenohypophysis of Phoxinus, Salmo and Cichlasoma but not in Carassius or Ameiurus. The cell types in this region of the pituitary are not histologically comparable in different species; the cellular changes in Phoxinus and Rana can be related to the release of melanophore concentrating- and dispersing hormones respectively but such a correlation has not proved possible for other species. Interpretations of the cytological appearance of the meta-adenohypophysis and intermediate lobe under different conditions are suggested and the functions of the chromatophore-regulating hormone are discussed.